Semiconductor process control technique



United States Patent Office Patented Nov. 9, 1965 3,216,084SEMICONDUCTOR PROCESS CONTROL TECHNIQUE Steward S. Flaschen and PrestonJ. Heinle, Phoenix, Ariz.,

assignors to Motorola, Inc., Chicago, 111., a corporation of Illinois NoDrawing. Filed Apr. 10, 1963, Ser. No. 271,867

6 Claims. (Cl. 2925.3)

This invention relates to semiconductor devices and particularly to amethod of adjusting certain specifications of transistors by controllingthe nature of the ambient atmosphere in which they are sealed.

In the manufacture of transistors it is often desirable to adjustcertain of the device characteristics to meet the specified needs ofparticular customers as well as to maintain the distribution of certaintypes of devices within the limits of their specifications. Suchadjustments are usually made by using semiconductor material ofdifferent characterisitics, adjusting geometric factors such as the basewidth of the device, or by altering major processing operations such asetching operations. These methods are troublesome, expensive and oftenrather slow since the feedback of information to optimize the processingadjustments may take from several hours to several days.

Accordingly, an object of the present invention is to provide a methodof adjusting transistor characteristics that is inexpensive, easy andrapidly accomplished.

The invention features encapsulating the transistors with varyingmixtures of drying agents and moistening agents to that the moisturelevel of the ambient atmosphere is maintained at appropiate levelsaccording to the distribution of transistor characterisitics desired.

This invention permits shifting of important device parameters such ascurrent gain, saturation currents or storage time at will within ratherbroad limits determined by device design and previous process variables.This is accomplished by controlling the moisture level within theambient atmosphere. The use of controlled ambient atmospheres iswell-known to the semiconductor industry to stabilize and optimizetransistor parameters but such systems in the past have provided arigidly controlled moisture level whereas the present invention providesany desired useful moisture level, and changes from one level to anothermay be easily carried out.

The invention provides for adjusting the moisture level by adjusting theratio of a reversible adsorbent such as molecular sieve in combinationwith varying amounts either of drying agents such as barium oxide (BaO)or moistening agents such as calcium sulfate hemihydrate (CaSO /zI-I O)and encapsulating these powders so formed with the transistors. Thereversible adsorbent is prepared in any convenient manner. For example,it is convenient to store opened containers of molecular sieve powder inovens at or near 200 C. After cooling in a dry atmosphere, the adsorbentis mixed with drying or moistening agents in the desired ratio. Theratio used determines the average value of the desired device parameterwhich will be obtained. The greater the proportion of drying agent, thedrier will be the ambient atmosphere. As a general rule, in PNPgermanium transistors, drier ambient atmospheres result in lower currentand power gains, higher saturation currents, and lower storage times inswitching applications. Thus, if high current gain is desired, a ratiois selected which will provide relatively more moistening agent thanwould be used with the adsorbent if lower current gains were de sired.Or, if a lower storage time is desired, a dry atmosphere is obtained forthis purpose by using adsorbent and drying agent together. Othercharacteristics, such as breakdown voltages, are not so stronglyaffected by the moisture level, though some difference may appear.

The purpose of the moistening or drying agents is primarily to vary theamount of moisture contained in the reversible adsorbent. It ispostulated that this amount of moisture determines the water vaporcontent of the encapsulated atmosphere in equilibrium with the powder inthe encapsulation, which in turn affects the device parameters.

The advantages of this invention are:

(1) The moisture content of the adsorbent may be controlled at anypreselected level, so that a continuous spectrum of controlled ambientmaterials is available.

(2) The powder is applied as one of the last steps in device production.If a need develops for more devices within a given range of parameters,a change in ratio may be used to shift the parameter distribution andthus provide the needed devices. All the devices, however, contain thesame materials; they are merely present in different ratios. Possibly"because the powders contain the same materials, the devices, unlessther parameters are widely different, have about the same agingcharacteristics at temperatures as high as 125 C.

. (3) The use of a good adsorbent, particularly molecular sieve,represents an attractive aspect in relation to reliability. While manyadsorbents react only or primarily with water, molecular sieve, forexample, can adsorb many other molecular and ionic species which may bepresent and may tend to degrade the device.

The manner of application of this process to a typical device, a powertransistor, is as follows: In normal production, industrial powertransistors are electrolytically etched after assembly. After etching,rinsing and drying, they are stored in hot ovens until just beforecanning (encapsulation). The canning operation is performed on a largewelding press. The cup-like can or lid for the transistor is placed inthe lower electrode of the welder and a charge of the ambient atmospherecontrolling materials is dispensed into it. The partial transistorassembly is then completed by placing it on the can, and the can is thenwelded to it.

Two alternate methods of dispensing the powder are used. In the firstmethod, a charge of hot (and therefore dry) molecular sieve is dispensedinto the can. Then a charge of cold calcium sulfate hemihydrate (CaSO/2H O) is dispensed into the can. The ratio between the two componentsis determined by controlling the sizes of the respective charges. In thesecond method, which is preferred, the components are premixed in theproper ratio and placed in the dispenser. No heating need be used, butthe chamber of the dispenser containing the mixture is flushed with aslow stream of dry, clean gas such as nitrogen or air to keep the powderfrom absorbing excess moisture. The appropriate volume of powder isdispensed into the can.

The second or preferred method provides better ratio control, moreuniform'composition of powder, and it is easier to change from one ratioto another.

The application of calcium sulfate hemihydrate-molecular sieve to aproduct line of industrial power transistors gave results as follows.The ratios used were (a) molecular sieve, (b) 0.5 part calcium sulfatehemihydrate to 1 part molecular sieve, (c) 3.5 parts calcium sulfatehemihydrate to 1 part molecular sieve and (d) 5.0 parts calcium sulfatehemidydrate to 1 part molecular sieve. The parameter measured was 1 with10 amps collector current (1 and 2 volts collector voltage, as a measureof the DC. cur-rent gain h since they are related by Specifications forthe device required an k to be within the limits to 30 which correspondstoamaximurn I of 1000 milliamps and a minimum of 333 milliamps. Groupsof ten transistors were canned each day for five consecutive days usingeach of the four listed-compositions giving a total of fifty devices foreach composition or treatment. The median values of 1 to reach groupwere as follows:

An analysis of variance shows with 99% confidence that groups (c) and(d) are not significantly different from each other and that groups (a)and (b) are dilferent from each other and from groups (c) and (d). Thus,in order to produce a maximum number of high h units (low 1 a ratio of3.5:1 or 5.0:1 should be used. If treatment (a) were used, about a thirdcould be expected to be outside specification for low gain while on theother hand powder (c) or ((1) would yield essentially 100% good unitsfor this parameter.

Other parameters were also affected but usually to a lesser extent.Diiferences between averages for breakdown voltages as measured fromcollector-to-base and emitter-to-base were not significant. The colectoror emit ter reverse currents varied with I but the difference wasgenerally less noticeable than the differences in I If still drierambients than those produced by the molecular sieve alone are desired,to establish lower h values (higher I a mixture of the molecular sieveand a dehydrating material such as barium oxide (BaO), in the properratio, can be used within the device enclosure. The system 'barium oxide(BaO)-molecular sieve was applied to a germanium mesa production line.Sixteen units were canned in each of five groups listed below, and themedian values h were calculated.

Group Ratio Thus, if one wishes to produce from a single lot oftransistors, a certain percentage of the production having levels of khigher than those produced by using barium oxide (BaO) alone, but lowerthan those produced by the use of the molecular sieve alone, one wouldemploy the group 3 or group 4 material (having a ratio of, 50:50 or25:75 BaOzmolecular sieve). The relative level of the parameter htherefore, can be preselected by the selection of the proper ratio ofthe two materials.

The invention is not limited to just one or a few types oftransistors'but operate on a great variety of device's 4 v usingradically dissimilar technologies. It is an inexpensive non-criticalprocess which is easy to apply where ever transistors are to beencapsulated in an ambient atmosphere.

We claim:

1. A method of controlling the electrical paraments of hermeticallysealed semiconductor devices by establishing a desired level of gaseousmoisture in the atmosphere within the devices, said method including thesteps of, mixing a reversible adsorbent material and a chemicallyreactive moisture controlling material in a predetermined ratio toprovide a mixture capable of establishing a preselcted moisture level insaid devices when sealed, placing said mixture in said devices, andsealing said devices.

2. A method of controlling the electrical parameters of hermeticallysealed semiconductor devices by establishing a desired level of gaseousmoisture in the atmosphere within the devices, said method including thesteps of placing in said devices a mixture, comprised of a predeterminedratio of a reversible adsorbent material and a chemically activemoisture controlling material capable of adjusting the moisture level insaiddevices when sealed to a level which establishes the electricalparameters of the devices at the desired values, and sealing saiddevices with said mixture therein. Y

3. The method of establishing a particular moistur level within anencapsulated semiconductor device, said method including the steps .of,mixing a reversible adsorbent material with a drying agent in apredetermined ratio to provide the particular moisture level, placingsaid mixture within said device, and sealing said device.

4. The method of establishing a particular moisture level within anencapsulated semiconductor device, said method including the steps of,mixing a reversible adsorbent material and a moistening agent in apredetermined ratio to provide the particular moisture level, placingsaid mixture within said device, and sealing said device.

5. The method of establishing a particular moisture level within anencapsulated semiconductor device, said method including the steps. of,mixing molecular sieve with calcium sulfate hemihydrate (CaSO /2H OD ina predetermined ratio to provide the particular moisture level, placingsaid mixture within said device, and sealing said device.

6. The method of establishing a particular moisture .level within anencapsulated semiconductor device, said method including the steps of,mixing molecular sieve with barium oxide (BaO) in a predetermined ratioto provide the particular moisture level, placing said mixture withinsaid device, and sealing said device.

References Cited by the Examiner UNITED STATES PATENTS RICHARD H. EANES,JR., Printary Examiner.

3. THE METHOD OF ESTABLISHING A PARTICULAR MOISTURE LEVEL WITHIN ANENCAPSULATED SEMICONDUCTOR DEVICE, SAID METHOD INCLUDING THE STEPS OF,MIXING A REVERSIBLE ADSORBENT MATERIAL WITH A DRYING AGENT IN APREDETERMINED RATIO TO PROVIDE THE PARTICULAR MOISTURE LEVEL, PLACINGSAID MIXTURE WITHIN SAID DEVICE, AND SEALING SAID DEVICE